Could not connect to the PostCSS server (RuntimeError)

I am running jekyll with jekyll-process through docker compose:

```
version: '3.3'

services:
jekyll:
image: jekyll/jekyll:latest
# command: jekyll serve --watch --force_polling --verbose --trace
command: jekyll serve --livereload --drafts --future --port 4000 --livereload_port 35729
ports:
- 4000:4000
volumes:
- .:/srv/jekyll
environment:
- JEKYLL_ROOTLESS=true
```
I keep getting error:

```
jekyll_1 | Configuration file: /srv/jekyll/_config.yml
jekyll_1 | jekyll 3.8.7 | Error: Could not connect to the PostCSS server
jekyll_1 | /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll-postcss/socket.rb:70:in `rescue in start_dev_server': Could not connect to the PostCSS server (RuntimeError)
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll-postcss/socket.rb:61:in `start_dev_server'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll-postcss/socket.rb:13:in `initialize'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll/converters/postcss.rb:15:in `new'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll/converters/postcss.rb:15:in `block in initialize'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll/converters/postcss.rb:15:in `fetch'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll/converters/postcss.rb:15:in `initialize'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:295:in `new'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:295:in `block in instantiate_subclasses'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:294:in `map'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:294:in `instantiate_subclasses'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:116:in `setup'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:32:in `initialize'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/build.rb:30:in `new'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/build.rb:30:in `process'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/serve.rb:93:in `block in start'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/serve.rb:93:in `each'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/serve.rb:93:in `start'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/serve.rb:75:in `block (2 levels) in init_with_program'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `block in execute'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `each'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `execute'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/program.rb:42:in `go'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary.rb:19:in `program'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/exe/jekyll:15:in `'
jekyll_1 | from /usr/local/bundle/bin/jekyll:29:in `load'
jekyll_1 | from /usr/local/bundle/bin/jekyll:29:in `'
jekyll_1 | /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll-postcss/socket.rb:62:in `initialize': Address not available - connect(2) for "localhost" port 8124 (Errno::EADDRNOTAVAIL)
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll-postcss/socket.rb:62:in `open'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll-postcss/socket.rb:62:in `start_dev_server'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll-postcss/socket.rb:13:in `initialize'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll/converters/postcss.rb:15:in `new'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll/converters/postcss.rb:15:in `block in initialize'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll/converters/postcss.rb:15:in `fetch'
jekyll_1 | from /usr/gem/gems/jekyll-postcss-0.4.1/lib/jekyll/converters/postcss.rb:15:in `initialize'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:295:in `new'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:295:in `block in instantiate_subclasses'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:294:in `map'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:294:in `instantiate_subclasses'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:116:in `setup'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/site.rb:32:in `initialize'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/build.rb:30:in `new'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/build.rb:30:in `process'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/serve.rb:93:in `block in start'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/serve.rb:93:in `each'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/serve.rb:93:in `start'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/lib/jekyll/commands/serve.rb:75:in `block (2 levels) in init_with_program'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `block in execute'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `each'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `execute'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/program.rb:42:in `go'
jekyll_1 | from /usr/gem/gems/mercenary-0.3.6/lib/mercenary.rb:19:in `program'
jekyll_1 | from /usr/gem/gems/jekyll-3.8.7/exe/jekyll:15:in `'
jekyll_1 | from /usr/local/bundle/bin/jekyll:29:in `load'
jekyll_1 | from /usr/local/bundle/bin/jekyll:29:in `'
```

Any idea how to resolve this?

Reproduce the failure with the Docker Compose configuration in the issue, then inspect lib/jekyll-postcss/socket.rb and lib/jekyll/converters/postcss.rb around the connection to localhost:8124. Compare the container setup with the plugin's expected PostCSS server behavior; done means the reported Jekyll command starts successfully without the connection error.

Written by the indexing model from the issue text.